1. Field of the Invention
The present invention relates to an overcurrent protection circuit for a dc-to-dc converter.
Such power supply devices as convert one direct-current voltage (hereinafter called an input voltage) into another direct-current voltage (hereinafter called an output voltage), which are called generally as a dc-to-dc converter, are divided broadly into a switching regulator and a series regulator. In order to obtain a required level of output voltage, the switching regulator switches the input voltage on and off by using a transistor, for example; and the series regulator drops the input voltage by using a resistor, for example.
The switching regulator-is divided further into what uses a high-frequency transformer and what directly switches the input voltage. Further, from the standpoint of input voltage-to-output voltage relationship, the switching regulator is classified into a voltage-step-up type and a voltage-step-down type, which convert the input voltage into higher and lower levels of output voltage, respectively.
Of those various types of dc-to-dc converters as mentioned above, it is known that a dc-to-dc converter which directly switches the input voltage on and off (called a ripple-control method) provides the highest conversion efficiency, if limited to a voltage-step-down type. Therefore, the dc-to-dc converter of the ripple control method is most suited for a power supply device for driving a light load, which power supply device requires especially high conversion efficiency.
In the field of information processing and communication, as demand increases for a highly reliable equipment, the same demand is also increasing for a power supply device in recent years. Accordingly, an overcurrent protection circuit for protecting a dc-to-dc converter from an overcurrent due to a short circuit caused in a load, is in great demand.
2. Description of the Related Art
FIGS. 1A-1C are circuit diagrams of prior-art overcurrent protection circuits for voltage-step-down type dc-to-dc converters of the ripple-control type.
In the dc-to-dc converter 210 shown throughout FIGS. 1A-1C, a field effect transistor (hereinafter abbreviated to FET) 211 is turned on and off according to the output of a comparator 212 so as to output a ripple voltage to a smoothing circuit. The smoothing circuit consisting of a zener diode 213, a choke coil 214 and a capacitor 215, smooths (or reduces the ripple of) the ripple voltage to provide a direct-current output voltage.
Resistor pairs R1 and R2 connected in series are grounded in parallel with the capacitor 215. The comparator 212 compares voltage Vd at the junction between the resistors R1 and R2 with reference voltage Vs and outputs a comparison result signal through a resistor R3 to the FET 211 gate, to which the input voltage is applied through a resistor R4. Thus, depending on whether or not the voltage Vd is lower than the reference voltage Vs, the comparator 212 provides the FET 211 gate with a low voltage level corresponding to logical "0" or an open-circuited state, respectively, thereby turning the FET 211 on and off and causing the FET 211 to output a ripple voltage. The ripple voltage is then smoothed by the smoothing circuit to provide stabilized output voltage.
In the dc-to-dc converter 210 which directly switches the input voltage with the FET 211 as explained above, if a load which is connected to the output of the dc-to-dc converter 210 is short-circuited, for example, causing an overcurrent, the overcurrent might flow into the FET 211, eventually damaging the dc-to-dc converter 210. To prevent this, the following method was conventionally used:
As shown in FIG. 1A, the overcurrent protection circuit has a fuse 501 inserted in the power input line 10 (line which supplies the input voltage to the dc-to-dc converter 210). When an overcurrent occurs,and flows through the fuse 501, the fuse 501 fuses and disconnects the power input line 10, thus preventing the overcurrent from flowing into the FET 211.
The overcurrent protection circuit shown in FIG. 1B has a current monitoring circuit 504 inserted in the power output line 20 (line through which the dc-to-dc converter 210 outputs the output voltage). The current monitoring circuit 504 includes a resistor RL and a comparator 502. The comparator 502 compares the voltage Vc across the resistor RL with the predetermined threshold voltage (not shown) and provides the comparison result signal to the comparator 212 so that the comparator 212 causes the FET 211 to turn off when the voltage Vc exceeds the threshold voltage due to a short circuit caused in the load, for example.
The overcurrent protection circuit shown in FIG. 1C has a voltage monitoring circuit 604 connected to the output of the dc-to-dc converter 210. The voltage monitoring circuit 604 includes resistors R5 and R6 and a comparator 601. The comparator 601 compares voltage at the junction between the resistors R5 and R6 with the predetermined reference voltage Vt and provides the comparison result signal the comparator 212 so that the comparator 212 causes the FET 211 to turn off when the former voltage becomes lower than the reference voltage Vt due to a short circuit in the load.
FIG. 2A is a circuit diagram of a prior-art overcurrent protection circuit for a series-regulator type dc-to-dc converter.
The series regulator 310 measures the output voltage with a voltage meter 311 and varies the resistance of a variable resistor 312 according to the measured voltage so as to drop the input voltage to a required level of output voltage. The overcurrent protection circuit has a fuse 501 inserted in the power input line 10 of the thus-constructed dc-to-dc converter. The fuse 501 fuses and disconnects the power input line 10 when an overcurrent occurs, thus protecting the series regulator 310 from the overcurrent.
FIG. 2B is a circuit diagram of a prior-art overcurrent protection circuit for a a voltage-step-up chopper type dc-to-dc converter.
The dc-to-dc converter 410 of a voltage-step-up chopper type which does not use a high frequency transformer, includes a choke coil 411, a smoothing circuit 412, a control circuit 414 and a switching circuit 503. The control circuit (CONT) 414 is responsive to the output of the smoothing circuit 412 for turning the switching element 413 on and off to provide a required level of output voltage.
As shown in FIG. 2B, the overcurrent protection circuit has a switching circuit 503 and the current monitoring circuit 504 as explained with FIG. 1B, inserted in the power input line 10 and power output line 20, respectively. The comparator 502 detects an output voltage drop resulting from a short circuit caused in the load, in the same way as explained with FIG. 1B and causes the switching circuit 503 to disconnect the power input line 10 so as to protect dc-to-dc converter from an overcurrent due to the short circuit.
However, as for the overcurrent protection circuit using the fuse 501 (see FIGS. 1A and 2A), a problem is that an overcurrent due to a short circuit caused in the load may not be able to fuse the fuse 501, if the dc power supply (not shown in the figures), which supplies the input voltage to the dc-to-dc converter, has a current limitation characteristics. That is, the dc power supply limits a current flowing therefrom to the dc-to-dc converter through the fuse 501, allowing the overcurrent to flow through the dc-to-dc converter, eventually destroying it.
As for the overcurrent protection circuit using the current monitoring circuit 504 for monitoring an output current with the resistor RL (see FIGS. 1B and 2B), it may surely detect a short circuit in the load and protect the dc-to-dc converter from destruction. However, a problem is that voltage drop caused by the resistor RL decreases the voltage conversion efficiency.
As for the overcurrent protection circuit shown in FIG. 1C, it is difficult to discriminate a so-called start-up state from a real load short-circuited state because the output voltage is low in either state. Hereinafter, the start-up state is defined as the state which occurs for the period from the time the input voltage is supplied to the dc-to-dc converter 210 until it starts a normal conversion operation to provide the normal output voltage. The load short-circuited state occurs after the start-up state (or start-up period) when a short circuit occurs in the load, causing a drop in the output voltage. Since the output of the dc-to-dc converter 210 has not yet reached the normal voltage level in the start-up period, a problem is that the comparator 601 will signal the comparator 212 to turn off the FET 211, eventually prohibiting the dc-to-dc converter 210 from performing the normal conversion operation.